In the packaging of fluids such as milk and other dairy products, extensive use is made of containers formed of paperboard sheet material having an overall coating of thermoplastic film such as polyethylene applied on the surfaces of the sheet. This plastic coated paperboard is impervious to moisture and is particularly suitable for use in packaging acidic products of which milk and orange juice are illustrative. The thermoplastic coating not only serves to moisture proof the container but also can be utilized for sealing the container closing flaps. This sealing action is accomplished by heating the surfaces to be sealed while pressing them tightly together to form a liquid tight plastic-to-plastic bond. In many such containers a spout having a pouring lip is enclosed within the closure portion of the container. When the container is sealed the sanitary integrity of the pouring lip is preserved until the container is opened. When the paperboard blank from which the container is formed is initially prepared, all of the surfaces of the container are coated with the thermoplastic material. The pouring lip is thus also coated, and in the event heat and pressure are applied evenly to all parts of the closure, it becomes tightly sealed within the closure. When such a container is opened the lip is apt to become damaged, resulting in objectionable operation and presenting an unsightly appearance. One object of the present invention is to provide for use with a container of plastic coated paperboard having a pouring lip sanitarily housed within a sealed enclosure, an improved method and apparatus for protecting this pouring lip from being permanently and inaccessibly sealed within the enclosure.
Another previously proposed method (described in U.S. Pat. No. 3,116,002) is the application of a non-adherent material to selected portions of the pouring spout. This method introduces the risk of misapplication of the material which could cause leaking seals or a hard-to-open pouring lip. The invention disclosed herein eliminates those risks, the cost of the non-adherent material, and the cost of application of that material.
A further major advantage of the present invention over previous methods for closing and sealing folded cartons is that it facilitates the efficient use of manufacturing equipment. It is normal practice in the folded carton manufacturing industry to utilize standard, rotary printing presses to make the blanks which are then folded into cartons. Such rotary printing presses normally have only a limited number (e.g. four) of color stands and therefore are capable of printing only a limited number of colors. In previous methods for closing and sealing folded cartons, it is necessary to utilize one of the color stands for printing an adhesive thereby leaving one less color stand available for printing colors. The present invention eliminates the necessity of printing an abhesive onto the carton blanks leaving all color stands available for printing colors. Thus, the present invention facilitates the manufacture of more colorful folded cartons using standard manufacturing equipment than previous methods, which can be a significant marketing advantage with respect to the products sold in such folded cartons.